1. Field of the Invention
The present invention relates generally to a stress sensor. More particularly, the present invention relates to a fiber optic sensor that monitors stress on a structure.
2. Background of the Invention
Embedding miniature sensors in structures, systems, storage and shipping containers, and other items allows the monitoring of these items to determine health, maintenance needs, lifetime, and other item characteristics. Information from miniature embedded stress sensors in a composite or other structure can tell a user information including whether or not the item has been dropped sufficiently to cause damage, stressed to a point that makes the integrity of the structure suspect, undergone more cyclic variations than it can handle, or has other health or performance issues.
Most current structures do not employ embedded sensors to aid in the determination of structural health. Instead, structures are assigned an operational lifetime that is significantly shorter than what could be achieved. In addition, structures are given a maintenance schedule and maintenance or disposal guidelines that are more frequent and strict than may be necessary. These procedures result in excess cost to an application and the user as structures are replaced, repaired, and maintained more often than necessary, while also using maintenance techniques and products that are more sophisticated and costly than they need to be. By using embedded sensors within structures, users can measure the actual health of the structure at a given point in time, regardless of the previous conditions seen by the structure, and then perform maintenance and replacement activities as necessary based on diagnosis of the structures' health rather than on a predetermined schedule.
To maintain reliability and integrity of the materials, sensors are often needed to monitor structure degradation while in storage, transport, or before use. A common method for detection of strain in a structure is to embed fiber optic sensors within the material. The two methods that are often used to detect damage utilize Fiber Bragg Gratings (“FBGs”) and etalons.
FBGs and the associated Bragg phenomenon have long been studied and implemented in numerous commercial applications, some of which have been related to structural health monitoring (“SHM”). A Fiber Bragg Grating is a periodic index of refraction gradient created in the optical fiber core. The grating allows only certain wavelengths to pass through the grating and others to be reflected back from the grating. Strains in the structure, and hence the optical fiber, alter the periodicity of the grating, thereby altering its transmission and reflection properties. These changes can be measured and stress concentrations can be calculated from the data.
In particular to SHM, FBGs have long been studied at a research level while some have been carried to the commercial availability stage. Regardless of the commercial readiness of a particular FBG, it can be described as a localized fiber optic sensor. In other words an FBG is a sensor that has a sensitive area that is dependent on the length of the grating that has been written on the optical fiber. A FBG cannot, for example, detect an applied strain that is not coincident or directly applied to the area of the optical fiber that contains the Bragg Grating. In many applications this is an acceptable operational procedure, while others may require a more distributed sensing technique.
One procedure to allow for multiple FBG sensing sites along a single optical fiber is to multiplex the FBGs together using more sophisticated instrumentation. Three problems often exist with this approach to achieve a partial distributed sensor: multiple sites along the fiber must be prepared to write the FBGs, sites of interest on the structure must be predetermined and matched to the location of the gratings, and more sophisticated and expensive instrumentation is required to interrogate multiple FBGs.
One concern that often accompanies the procedure to embed FBGs into a structure is that in many cases the protective acrylate coating of the optical fiber has to be removed in order to write the FBG. The removal of the protective coating requires special procedures to minimize potential damage due to handling and or environmental conditions. These procedures can often restrict the type of environment that can implement a FBG or require a protective device that is not allowable in the structure to be monitored.
Similar in sensor arrangement to FBGs are Fabry-Perot Etalons (“FPE”). Etalons are a point source detection scheme that can measure stress applied to the structure that the FPE is embedded in. Etalons have been shown to have exceptional characteristics for detecting stress in a matrix and indicating relative levels of that stress. To be useful and to ensure optimum sensitivity, however, the potential locations for damage must be identified before the etalons are placed in the structure. Also, as is the case with FBGs, the protective coating on the fiber near the etalon is most often removed to improve sensitivity of the sensor. This removal of material drastically decreases the robustness of the sensor, which could limit its end application for use in hostile environments.
It would therefore be desirable to have a fiber optic stress sensing system that overcomes the limitations of FBGs and FPEs and offers a more robust and cost effective sensor that can be installed in a distributed fashion over a structure without requiring pre-designation of areas of potential stress.